The heat-treatment of pellets, particularly the firing of iron ore pellets to harden them is effected in most cases on traveling grates which are provided with gas hoods and described as pellet-firing machines. Such pellet-firing machines consist of several zones, which succeed each other in the direction of travel, namely, a drying zone, a heat-treating zone and a cooling zone. These zones may be subdivided, for instance, into predrying and final drying sections, a preheating section, a preliminary firing section, a main firing section and an afterfiring section, and first and second cooling zone sections. In most cases, all or most of the process heat which is required is introduced into the process by hot gases. These hot gases are generated in gas hoods provided over the pellet bed by a combustion of liquid, gaseous or dustlike solid fuels or they are collected and distributed by such hoods. As part of the exhaust gases are very hot, various gas-recycling systems are used for utilization of heat.
Such a pellet-firing machine is known from German Patent Specification 1,433,339. In that machine, hot cooling gas from an updraft first cooling zone section is conducted in a common gas hood without an interposed blower into the heat-treating zone, which consists of preheating, firing and afterfiring sections. The cooling gas is distributed to the several sections of the heat-treating zone by means of internal fixtures provided in the common gas hood. These internal fixtures define passages leading to the actual combustion chambers of the several zone sections. In the combustion chambers of the preheating and firing sections, the hot cooling gases are heated up by burners to the required temperature. The hot gases are sucked through the bed into windboxes. Gases from the second cooling zone section and exhaust gases from the afterfiring section are fed into the drying zone sections. It is also described that the hot cooling gases from a gas hood disposed over the cooling zone are withdrawn through a gas manifold and are distributed via distributing ducts to the several sections of the heat-treating zone.
In a similar pellet-firing machine known from U.S. Pat. No. 3,620,519, the hot cooling gases are heated up by burners in the common gas hood over the cooling zone and/or in the transition zone between the cooling and heat-treating zones. An internal fixture for shielding the pellets from the hot combustion gases is installed at least in part of the transition zone in the common gas hood disposed over the pellet bed. The use of burners to supply all heat that is required may give rise to an occurrence of hot spots, where ash from the fuel and/or dust contained in the process gases may be transformed into slag and form crusts adjacent to the burners. These crusts may deflect the flames or may permit an infiltration of ash whereby the refractory material may be destroyed. This may also adversely effect the heat treatment or may decrease the throughput or may necessitate repairs requiring the plant to be shut down. Besides, pellet-firing machines provided with burners require a very large number of burners so that these machines are mainly desirable for the use of gaseous and liquid fuels, which are relatively expensive. Where pulverized-coal burners are used, it is desirable to decrease the number of burners because special means are required to convey and distribute the fuel.
It is also known to incorporate part of the required fuel in the pellets although only a small part of the fuel can be incorporated without adversely affecting the quality of the pellets.
It is an object of the invention to avoid or substantially to decrease the occurrence of hot spots and the resulting problems due to slagging and decreased durability. Besides, it is also desirable to permit the use of inexpensive fuel and to provide a process with which the operation of existing plants can be improved.